Applications of bound states in the continuum in photonics

Bound states in the continuum (BICs) have attracted attention in photonics owing to their interesting properties. For example, BICs can effectively confine light in a counterintuitive way, and the far-field radiation of photonic structures that exhibit BICs has fascinating topological characteristics. Early research into photonic BICs was primarily focused on designing artificial structures to produce BICs. However, since the mid-2010s, exploring the potential applications of BICs has been a growing trend in research. In this Review, we detail the unique properties of BICs, including the ability to achieve enhanced light confinement, sharp Fano resonances and topological characteristics. We explore phenomena derived from BICs, including the generation of circularly polarized states and unidirectional guided resonances, and the impact of BICs on various applications such as lasing, nonlinear frequency conversion, waveguiding, sensing and wavefront control. We also discuss the insights provided by BICs in several emerging research frontiers, such as parity-time symmetric systems, higher-order topology, exciton-photon coupling and moire superlattices. Photonic systems provide a versatile platform to explore and use bound states in the continuum. This Review discusses the potential of these states for enhancing light-matter interactions in various applications and investigating the physics of emerging photonic systems. Photonics provides a versatile platform to study and exploit the properties of bound states in the continuum (BICs), leading to a wide range of applications.The ability of BICs to achieve highly efficient light confinement leads to the coherent field enhancement of both electric and magnetic fields, which can improve lasing performance, nonlinear conversion efficiency and waveguiding in photonic integrated circuits.Light scattered by photonic structures exhibiting BICs manifests the features of Fano resonances, leading to advanced functionalities in refractometric sensing and the identification of molecular fingerprints.BICs are characterized as topological polarization vortices in far-field radiation, in which the geometric phases in momentum space can be used to manipulate light, giving rise to polarization conversion, vortex beam generation and beam shifts.BICs have been integrated to explore several emerging frontiers, including parity-time-symmetric systems, higher-order topology, exciton-photon coupling and moire superlattices.